There is nothing illegal about converting a Ruger 10/22 to select fire.
Some of the replies here are not correct.
What you need is called a Norell Trigger pack. You need to search the boards that cater to NFA weapons to find one for sale. Onw good one would be
www.subguns.com. They appear fairly frequently. The conversion is very simple. You install the trigger pack just like you would install any trigger: just remove the two pins for the trigger pack, put the new trigger pack in it's place and put the two pins back in place.
I did a quick search on subguns.com and came up with this:
http://www.subguns.com/classifieds/...sults_format=long&db_id=13496&query=retrieval
I am not familiar with the trigger, but it is a full auto 10/22.
This is the website for the guy that made the Norell Triggers (John Norell):
http://www.johnnorrellarms.com/default.asp
Due to federal law, he had to stop manufacturing new ones in 1986 however there are still used ones floating around.
I have never owned or fired one, but I have heard that it was possibly the best full auto in .22LR ever made including the American 180.
You might want to consider an M16 registered reciever. It is much more versitile and doesn't cost all that much more. You can put various uppers on it to give you anything from a .22LR to a 9mm, to a .223, to a .458 SOCOM or whatever your heart desires. There are some good .22LR uppers out there from companies like Spike's Tactical and some good .22LR magazines as well.
I have always wanted a Norell Trigger pack but I think from a practical standpoint, the M16 RR is the way to go.